The present invention relates to a vapor detector for measuring vapor transmission rate characteristics of sheet materials; more particularly, the invention relates to determining the vapor transmission characteristics of sheet materials online, during a manufacturing process. The present invention is also useful for measuring vapor transmission characteristics of materials offline, by way of comparing characteristics of two or more different samples of the material, in a very quick procedure.
Certain apparatus for measuring vapor flow rate using particular sensor devices are known in the prior art. One type of sensor device typically provides an electrochemical transformation in response to the presence of the vapors being measured, as for example, measuring oxygen permeating through a membrane. One such detector is disclosed in U.S. Pat. No. 3,223,597, issued Dec. 14, 1965, and another form of oxygen detector is disclosed in U.S. Pat. No. 4,085,024, issued Apr. 18, 1978. These devices utilize a glass envelope which encloses an anode overlaid with nickel-cadmium (Ni--Cd), wrapped with an insulating material, and having a carbon fiber cathode overlying the insulating material. Electrical conductors attached to each of the anode and cathode are brought to the outside of the glass envelope through sealed openings. A gas inlet and gas outlet is provided through the glass envelope, to permit the flow-through passage of an oxygen-carrying gas. The interior of the glass tube is filled or partially filled with potassium hydroxide (KOH). The oxygen molecules in the gas cause an electrochemical reaction to occur between the anode and cathode, whereby a small current is developed through the conductors, and the magnitude of the current is representative of the oxygen content of the gas.
Other types of sensors utilize predetermined wavelengths of radiated energy to detect the presence of certain vapors or gases, as for example, infrared detectors for detecting the presence of water vapor and the like.
It is usually desirable to make measurements with these devices under equilibrium conditions, where a vapor or gas source is confined in a chamber on one side of a membrane, and a vapor or gas detector is connected to a chamber on the other side of the membrane, and conditions are held constant until the vapor or gas permeating through the membrane reaches a constant level, where as much vapor or gas leaves the downstream side of the membrane as enters the upstream side of the membrane. The test for making this measurement necessarily takes a considerable amount of time, as it can take many hours for the conditions to stabilize.
It is also possible to measure the vapor or gas transmission characteristics by a process that measures the outgassing characteristics of the material. In this circumstance, the material is first saturated in the gas or vapor of interest, and then is placed in a closed chamber where the vapor which outgasses from the material can be measured. The rate at which the vapor outgasses provides a measure of the vapor transmission characteristics of the material. An apparatus and method which describes this technique is found in my U.S. Pat. No. 5,591,898, issued Jan. 7, 1997, and entitled "Method for Measuring Material Permeability Characteristics."
It is also possible to measure the vapor or gas transmission characteristics of a material by a process that measures the outgassing characteristics of the material, even if the material has not been totally saturated in the gas or vapor of interest. This process is possible because the outgassing characteristics of any material follow a predictable complex exponential curve over time, and partial saturation of a material causes outgassing to occur along the same curve as complete saturation of the material; collection of only a few data points along this curve enables one to reconstruct the outgassing curve as though the material had been completely saturated.
Each of the foregoing processes provide a technique for obtaining a measure of transmission rate of a particular gas or vapor through a particular material, and therefor provides a quantitative value for the gas or vapor transmission rate of the particular material. This information is valuable in enabling a manufacturer to select, for example, the proper materials for packaging particular foods, liquids, medicines, etc., where contamination of the product within a package, by gases or vapors passing through the package material, may degrade or spoil the product. However, each of the foregoing processes require some time to complete, at least several hours, and therefore can only be practically performed under off-line conditions where the packaging materials may be tested on a statistical sampling basis. It would be extremely desirous if the transmission rate characteristics of a material could be obtained on-line; i.e., while the material is being manufactured or while the product packaging process is being performed. Furthermore, it would be desirous if the measurement of transmission rate characteristics of materials being manufactured could serve as a qualitative standard to regulate the manufacture of the packaging material, or by monitoring the vapor barrier qualities of a packaging material while the product packaging operation proceeds, wherein the manufacturing line could be shut down whenever the vapor barrier qualities of the packaging material on the line deviates outside a predetermined range of desired transmission rate characteristics.